Kathryn Day (née Bouleyn) is an American opera singer who has had an active international career spanning five decades. She began her career as a leading soprano under the name Kathryn Bouleyn in the 1970s and 1980s with companies like the New York City Opera, the San Francisco Opera, and the Opera Theatre of Saint Louis. With the latter institution she created the role of Cora in the world premiere of Stephen Paulus' The Postman Always Rings Twice (1982).
In the 1990s Day transitioned to leading mezzo-soprano roles with the aid of companies like the Seattle Opera and the Opéra de Montréal. In 2003 she portrayed Jeanne Loiuse de Pontalba in the world premiere of Thea Musgrave's Pontalba at the New Orleans Opera. She has been a regular presence at the Metropolitan Opera in mainly comprimario roles since 2005. She has appeared on several broadcasts of the Metropolitan Opera Live in HD. She has sometimes been billed as Kathryn Bouleyn Day.